The invention relates to luminescent compounds, and more particularly to luminescent compounds based on squaric, croconic, or rhodizonic acid, among others.
A luminescent compound, or luminophore, is a compound that emits light. A luminescence method, in turn, is a method that involves detecting light emitted by a luminophore, and using properties of that light to understand properties of the luminophore and its environment. Luminescence methods may be based on chemiluminescence and/or photoluminescence, among others, and may be used in spectroscopy, microscopy, immunoassays, and hybridization assays, among others.
Photoluminescence is a particular type of luminescence that involves the absorption and subsequent re-emission of light. In photoluminescence, a luminophore is excited from a low-energy ground state into a higher-energy excited state by the absorption of a photon of light. The energy associated with this transition is subsequently lost through one or more of several mechanisms, including production of a photon through fluorescence or phosphorescence.
Photoluminescence may be characterized by a number of parameters, including extinction coefficient, excitation and emission spectrum, Stokes"" shift, luminescence lifetime, and quantum yield. An extinction coefficient is a wavelength-dependent measure of the absorbing power of a luminophore. An excitation spectrum is the dependence of emission intensity upon the excitation wavelength, measured at a single constant emission wavelength. An emission spectrum is the wavelength distribution of the emission, measured after excitation with a single constant excitation wavelength. A Stokes"" shift is the difference in wavelengths between the maximum of the emission spectrum and the maximum of the absorption spectrum. A luminescence lifetime is the average time that a luminophore spends in the excited state prior to returning to the ground state. A quantum yield is the ratio of the number of photons emitted to the number of photons absorbed by a luminophore.
Luminescence methods may be influenced by extinction coefficient, excitation and emission spectra, Stokes"" shift, and quantum yield, among others, and may involve characterizing fluorescence intensity, fluorescence polarization (FP), fluorescence resonance energy transfer (FRET), fluorescence lifetime (FLT), total internal reflection fluorescence (TIRF), fluorescence correlation spectroscopy (FCS), fluorescence recovery after photobleaching (FRAP), and their phosphorescence analogs, among others.
Luminescence methods have several significant potential strengths. First, luminescence methods may be very sensitive, because modern detectors, such as photomultiplier tubes (PMTs) and charge-coupled devices (CCDs), can detect very low levels of light. Second, luminescence methods may be very selective, because the luminescence signal may come almost exclusively from the luminophore.
Despite these potential strengths, luminescence methods suffer from a number of shortcomings, at least some of which relate to the luminophore. For example, the luminophore may have an extinction coefficient and/or quantum yield that is too low to permit detection of an adequate amount of light. The luminophore also may have a Stokes"" shift that is too small to permit detection of emission light without significant detection of excitation light. The luminophore also may have an excitation spectrum that does not permit it to be excited by wavelength-limited light sources, such as common lasers and arc lamps. The luminophore also may be unstable, so that it is readily bleached and rendered nonluminescent. The luminophore also may have an excitation and/or emission spectrum that overlaps with the well-known autoluminescence of biological and other samples; such autoluminescence is particularly significant at wavelengths below about 600 nm. The luminophore also may be expensive, especially if it is difficult to manufacture.
The invention provides photoluminescent compounds, reactive intermediates used to synthesize photoluminescent compounds, and methods of synthesizing and using photoluminescent compounds, among others. X
The compounds relate generally to the following structure: 
Here, Z is a four, five, or six-member aromatic ring, and A, B, C, D, E, and F are substituents of Z, where F is absent when Z is a five-member ring, and where E and F are absent when Z is a four-member ring. Generally, A, B, C, D, E, and F may be present in any order, although the order may be limited in certain embodiments.
A, B, C, D, E, and F are selected from a variety of elements and groups, including but not necessarily limited to O, S, Se, Te, C(Ra)(Rb), Nxe2x80x94Rc, N(Rd)(Re), W1, and W2. 
The components Ra, Rb, Rc, Rd, Re, n, X1, X2, X3, X4, and Y are defined in detail in the Detailed Description. However, generally, each compound includes at least one of W1 or W2, with the preferred synthetic precursors including one, and the preferred photoluminescent compounds including two. In some embodiments, the compound includes at least one S. In other embodiments, the compound includes at least one heteroatom in X1 through X4 of W1 or W2. In yet other embodiments, the compound includes a reactive group and/or a carrier. In yet other embodiments, A, B, C, D, E, and F are chosen so that the compound is photoluminescent.
The methods relate generally to the synthesis and/or use of photoluminescent compounds, especially those described above.
The nature of the invention will be understood more readily after consideration of the drawing, chemical structures, and detailed description of the invention that follow.